Prepare changelog for 0.9.1.0

Also, add mutableByteArrayAsForeignPtr for symmetry with
byteArrayAsForeignPtr.

Data/Primitive/ByteArray.hs

@@ -63,6 +63,7 @@ module Data.Primitive.ByteArray (
   isByteArrayPinned, isMutableByteArrayPinned,
 #endif
   byteArrayAsForeignPtr,
+  mutableByteArrayAsForeignPtr,
   byteArrayContents,
   withByteArrayContents,
   mutableByteArrayContents,
@@ -130,11 +131,21 @@ newAlignedPinnedByteArray (I# n#) (I# k#)
 
 -- | Create a foreign pointer that points to the array's data. This operation
 -- is only safe on /pinned/ byte arrays.  The array's data is not garbage
--- collected while references to the foreign pointer exist.
+-- collected while references to the foreign pointer exist. Writing to the
+-- array through the foreign pointer results in undefined behavior.
 byteArrayAsForeignPtr :: ByteArray -> ForeignPtr Word8
 {-# INLINE byteArrayAsForeignPtr #-}
 byteArrayAsForeignPtr (ByteArray arr#) = ForeignPtr (byteArrayContents# arr#) (PlainPtr (unsafeCoerce# arr#))
 
+
+-- | Variant of 'byteArrayAsForeignPtr' for mutable byte arrays. Similarly, this
+-- is only safe on /pinned/ mutable byte arrays. This function differs from the
+-- variant for immutable arrays in that it is safe to write to the array though
+-- the foreign pointer.
+mutableByteArrayAsForeignPtr :: MutableByteArray RealWorld -> ForeignPtr Word8
+{-# INLINE mutableByteArrayAsForeignPtr #-}
+mutableByteArrayAsForeignPtr (MutableByteArray arr#) = ForeignPtr (mutableByteArrayContents# arr#) (PlainPtr arr#)
+
 -- | Yield a pointer to the array's data. This operation is only safe on
 -- /pinned/ byte arrays. Byte arrays allocated by 'newPinnedByteArray' and
 -- 'newAlignedPinnedByteArray' are guaranteed to be pinned. Byte arrays

changelog.md

 ## Changes in version 0.9.1.0
-  * Add `byteArrayAsForeignPtr`.
+
+  * Make fromListN functions good consumers for list fusion.
+
+  * Add functions to improve `MutVar`'s interoperability with `IORef` and `STRef`.
+
+  * Add `createPrimArray` and `createByteArray`.
+
+  * Add `byteArrayAsForeignPtr` and `mutableByteArrayAsForeignPtr`.
+
+  * Use `copyMutableByteArrayNonOverlapping#` in the implementation of `copyMutableByteArray`
+    on sufficiently new GHCs. This does not change the contract for `copyMutableByteArray`.
+    This function has always been documented as having undefined behavior when the slices
+    overlap. However, overlaps previously were handled gracefully (with the semantics
+    of C's `memmove`). Going forward, users who do not uphold `copyMutableByteArray`'s
+    precondition will be met with unpredictable results.
+
+  * Drop support for GHC 8.0.
 
 ## Changes in version 0.9.0.0